Modern radar systems rely heavily on digital processing techniques for improved performance in the accomplishment of certain functions which are unduly difficult or even impossible using purely analog techniques.
The invention applies in particular to radar systems such as so-called Doppler MTI systems in digital form where it is desired to accept only moving targets or to separate echo signals received in response to pulse transmissions on the basis of the Doppler modulation thereon.
Digital MTI radar systems per se are well known in the art and have been extensively described in the patent and other technical literatures. The text Radar Handbook by Merrill I. Skolnik (McGraw Hill Book Company, 1970) describes this background knowledge particularly in Chapter 17 thereof. The so-called bipolar video output modulated by the aforementioned Doppler frequency component will be seen in FIG. 6 presented in the aforementioned Chapter 17 of the Skolnik text. The change in path length between the radar equipment and the moving reflecting object produces a phase change from pulse to pulse, the rate of this change being a function of the Doppler frequency which, in turn, is directly related to the radial velocity of the reflecting object with respect to the radar equipment. The term "bipolar" is used to indicate that the coherent video pulses corresponding to successive transmitted pulses vary at the Doppler rate between a nominal maximum positive value and a nominal maximum negative value, the rate of that variation being directly attributable to the Doppler modulation component aforementioned.
Whatever means of angular beam scan are employed, it is normal to have a number of "hits" on the object or target of interest, those hits each corresponding to a positive or negative amplitude in the corresponding bipolar video signal within the maximum positive and negative amplitude limits. The number of hits is the number processed as a batch by the filter.
In a digital system of the type to which the present invention applies, the value within the bipolar video is encoded digitally, that is, a discrete digital number is generated representative of its instantaneous peak amplitude. The aforementioned process for generating digital coherent video signals is of itself well known and is extensively described in the patent and other technical literatures. For example, U.S. Pat. No. 3,406,396 includes a description of the aforementioned encoding process of a complex video signal. The term "complex video," is as understood in this art, viz., two signals called I and Q components (real and imaginary terms) result from coherent detection against zero phase and 90.degree. phase coherent oscillator references. It is complex video signals of this type to which the combination of the present invention is intended to respond.
In a purely analog arrangement, so-called "comb" filters have been employed to separate received echo signals on the basis of their Doppler modulation and, therefore, of their corresponding radial velocities. Such filters and variations thereof can be understood as background information,, also from the patent and other technical literature; for example, the text Modern Radar by Raymond S. Berkowitz (John Wiley and Sons, 1965, Third Printing 1967, and further identified by Library of Congress Card No. 65-21446). Chapter 2 of that text entitled "MTI Radar Filters" is of particular interest in connection with the background of MTI and pulse Doppler radar system filters.
Of more immediate interest as prior art, it will be noted that digital narrowband filterbanks have been constructed, and these have provided significant processing improvement over wider band filters because of coherent gain (favorable S/N) and independent control of the individual filters. Such prior filterbank arrangements may employ fast Fourier transform (FFT) and optimum filter (OF) techniques. These existing digital filterbanks usually employ a plurality of fixed digital filters, each of those filter modules responding to a set of discrete Doppler frequencies within the expected range of frequencies. Obviously, the narrower the frequency response of these individual filter modules, the more of them are required. A compromise in favor of somewhat wider Doppler filter bandwidths reduces the numerical requirement for filter modules, but also reduces the signal processing gain and the accuracy of target velocity determination. Still further, a fixed or non-adaptive filter module produces a non-fault-tolerant arrangement in which a filter response may be totally absent for one or more discrete Doppler frequencies. Moreover, whatever controlling circuitry is employed, introduces an additional element of failure susceptibility.
The manner in which the present invention deals with the prior art disadvantages to provide a new adaptive, fault-tolerant narrowband filterbank will be understood as this description proceeds.